Bug 1575369: Fix test race. r=mccr8

Differential Revision: https://phabricator.services.mozilla.com/D43288

--HG--
extra : moz-landing-system : lando

browser/components/extensions/test/browser/browser_ext_sidebarAction_incognito.js

@@ -3,7 +3,7 @@
 "use strict";
 
 add_task(async function test_sidebarAction_not_allowed() {
-  SpecialPowers.pushPrefEnv({
+  await SpecialPowers.pushPrefEnv({
     set: [["extensions.allowPrivateBrowsingByDefault", false]],
   });